The goal of the proposed research is to establish the presence of a recombination system in the mitochondria of animal cells. There is abundant genetic and physical evidence for recombination in the mitochondria of lower eucaryotes. There has been less opportunity to study recombination in animal cell mitochondria. However, there is some evidence from somatic cell hybrids of linkage between two species of mitochondrial DNA (mouse and human). By analyzing the restriction pattern of mitochondrial DNA from somatic cell hybrids constructed from both closely and distantly related species, the question of recombination will be studied in more detail. The Southern blotting technique and hybridization with highly labelled mitochondrial DNA probes will allow for increased sensitivity and resolution from that utilized in previous studies of this kind. An attempt will be made to construct cell lines with deletions in the mitochondrial genome by selective incorporation of BUdR into mit-DNA and irradiation. Deletions will provide the means for studying in a homologous system, recombination and transmission of mitochondrial markers and the establishment of homogeneity in the mitochondrial genome of animal cells.